1. Field of the Invention
The present invention relates to a detector for detecting an electromagnetic wave (THz-wave) in a THz-frequency band and more particularly to a bolometer-type THz-wave detector.
2. Description of the Related Art
Recently, an electromagnetic wave in a terahertz frequency band between light and an electromagnetic wave (that is, an electromagnetic wave with a frequency of 1012 Hz and a wavelength of approximately 30 μm to 1 mm, hereinafter referred to as THz-wave) has drawn attention as the electromagnetic wave directly reflecting information of a substance. A detector for detecting the THz wave (hereinafter referred to as a THz-wave detector) is generally in a structure comprising an antenna portion such as a dipole antenna or a Bow-tie antenna capturing the THz wave and an electric signal conversion portion for converting the THz wave captured by the antenna portion into an electric signal. As methods of converting the electromagnetic wave into the electric signal, Capacitive coupling method, Resistive coupling method and the like are known.
U.S. Pat. No. 6,329,655 discloses, for example, a Capacitive-coupling type THz-wave detector as shown in FIGS. 16A and 16B. This THz-wave detector is in a structure in which a glass layer 21 is formed on a substrate 20, four metal antennas 22 (Bow-tie antennas) are formed on the glass layer 21, and a detecting element 27 in which a heater film 23, an insulator 24, a thermal resistance layer 25, and an insulator 26 are laminated is formed at the center part of the four metal antennas 22 with a predetermined gap (GAP 1 and GAP 2) from the glass layer 21 and the metal antennas 22.
U.S. Pat. No. 6,329,655 also discloses a Resistive-coupling type THz-wave detector as shown in FIGS. 17A and 17B. This THz-wave detector is in a structure in which the glass layer 21 is formed on the substrate 20, the four metal antennas 22 (Bow-tie antennas) are formed on the glass layer 21, and the detecting element 27 in which the heater film 23 connected to the four metal antennas 22 is formed with a predetermined gap (GAP 3) from the glass layer 21 and the insulator 24, the thermal resistance layer 25, and the insulator 26 are laminated on the heater film 23 is formed at the center part of the four metal antennas 22. In the structure of the Resistive-coupling type THz-wave detector, a leg 28 with an impedance matched to 50 to 100Ω is needed in order to effectively transmit energy collected by the metal antennas 22 to the heater film 23, and heat conductance becomes large. Therefore, it is described that sensitivity of the Resistive-coupling type THz-wave detector is lower by one order of magnitude than that of the Capacitive-coupling type THz-wave detector.
In the case of detection of a THz wave by the Capacitive-coupling type THz-wave detector, efficient transmission of energy collected by the metal antennas 22 to the heater film 23 is required. For that purpose, a gap between the glass layer 21 and the detecting element 27 (GAP 1), and a gap between the metal antenna 22 and the detecting element 27 (GAP 2) should be controlled accurately. The above U.S. Pat. No. 6,329,655 describes that a scope of 0.1 to 1 μm is preferable as the value of GAP 2. However, if the detecting element 27 is to be floated from the glass layer 21 by the leg 28 using the MEMS (Micro-Electro-Mechanical Systems) technology, it is difficult to set the gap within the range of 0.1 to 1 μm, and there is a problem that yield is lowered.
In the case of detection of the THz wave by the Resistive-coupling type THz-wave detector, efficient transmission of the energy collected by the metal antennas 22 to the heater film 23 is also required. For that purpose, the gap between the glass layer 21 and the heater film 23 (GAP 3) should be controlled accurately. The above U.S. Pat. No. 6,329,655 describes that a scope of 0.2 to 1 μm is preferable as the value of GAP 3. However, if the heater film 23 is to be floated from the glass layer 21 using the MEMS technology, it is difficult to set the gap within the range of 0.2 to 1 μm, and there is a problem that yield is lowered.
It is also known that an effective aperture that can capture the electromagnetic wave by the antenna becomes an area merely of a circle with a radius of a half wavelength at the most. It is necessary to increase the size of the metal antenna 22 to efficiently capture the THz wave, but if the THz-wave detector in the above structure is made into a two-dimensional array, the size of each detector is limited. Therefore, the size of the detecting element 27 inevitably becomes small. For example, with the THz wave with the wavelength of 1 mm, the size of the detecting element 27 is approximately several μm. It is extremely difficult to incorporate the detecting element 27 in such a small region of several μm, and there is a problem that the yield is further deteriorated.